In the related art, as a bearing used for a high-speed rotating body, a thrust bearing is known which is disposed facing a thrust collar provided on a rotary shaft. As such a thrust bearing, a foil-type thrust bearing, namely a thrust foil bearing is well known. The bearing surface of the thrust foil bearing is formed of a flexible foil (a metal thin sheet) in order to absorb movement (axial displacement and inclination of the thrust collar) of the rotary shaft caused by vibration or impact, and the thrust foil bearing includes a foil structure which is provided under the bearing surface and flexibly supports the bearing surface.
As an example of such a thrust foil bearing, a structure is known in which a circular ring-shaped (annular shaped) bearing surface is formed of a plurality of individual piece-shaped foil pieces (top foil pieces) obtained by dividing an annular sheet in the circumferential direction thereof, and wave sheet-shaped foil pieces (bump foil pieces) support the top foil pieces (for example, refer to Patent Document 1). In addition, in such a thrust foil bearing, a thrust bearing is known in which each top foil piece (the thickness thereof is about 100 μm) is disposed at an inclination angle with respect to the thrust collar, and thereby the bearing clearance between the thrust collar and the top foil piece is formed in a wedge shape in side view. That is, the bearing clearance is formed so as to gradually decrease from the leading side toward the trailing side of the bearing clearance in the rotational direction of the thrust collar (the rotary shaft). Therefore, when the thrust collar rotates from the large side (the leading side) toward the small side (the trailing side) of the bearing clearance, a lubricating fluid flows into the wedge-shaped bearing clearance, and the load capability of the thrust bearing is obtained.
Only the edge on the leading side of the top foil piece in the rotational direction of the thrust collar (the rotary shaft) is fixed to a base plate, and the edge is configured as a fixed edge. When a bearing load increases, the top foil piece moves to be horizontal (parallel to the supported surface of the thrust collar) in a state where the fixed edge (the edge on the leading side) serves as a fulcrum, the inclination angle thereof decreases, and when the inclination angle becomes about 0.1°, the thrust bearing generates the maximum load capability. In addition, the bump foil piece is disposed so that the ridge line of a peak thereof is parallel to the edge on the trailing side of the top foil piece, and only the edge on the trailing side of the bump foil piece in the rotational direction of the thrust collar (the rotary shaft) is fixed to the base plate. That is, the edge on the leading side of the bump foil piece is configured as a free end.
The bump foil piece is disposed and fixed in this way because the pressure of a fluid lubrication film generated at the top foil piece becomes high at the small side (the trailing side) of the bearing clearance, and if this part is supported with a high rigidity, the load capability can be improved.
In addition, Patent Document 2 discloses a thrust foil bearing in which each top foil piece is supported by a wave sheet-shaped foil piece (a bump foil piece).
Furthermore, Patent Documents 3 to 5 disclose thrust foil bearings including a top foil and a back foil (a bump foil). Patent Document 6 discloses a radial foil bearing including a top foil and a back foil.